The United States National Security Agency (NSA) has admitted that its computer systems suffered a "serious" breakdown last week, badly affecting the processing of intelligence data.

The ultra-secretive agency said its computers went down on Monday evening at its headquarters in Fort Meade, Maryland, and the problem was not resolved until Thursday.

The NSA is responsible for protecting US information systems and specialises in gathering intelligence information through satellites, telephone intercepts and other methods.

The agency said late on Saturday that its computers were now operating "within the window of normal operations".

But it said it had taken technicians thousands of man-hours and $1.5m to get the computers up and running again.

The crash "did not affect intelligence collection, but did affect the processing of intelligence information", the agency said in a statement.

Contingency

The NSA said it had immediately put contingency plans into effect when the crash happened so that "other aspects" of the computer system could assume some of the load.

"While intelligence collection continued, NSA technicians worked to recover the information technology infrastructure."

"The backlog of intelligence processing is almost complete, and NSA is confident that no significant intelligence information has been lost."

Satellite failure

This latest incident follows the failure on New Year's Eve of a critical US spy satellite system as the result of a Y2K glitch.

The US Defense Department acknowledged that the flow of information streaming in from space was interrupted for several hours.

However the Pentagon insisted that that incident had not put national security in jeopardy.